The invention relates to packers for use in well bores which are to be subjected to high temperatures. During a multiple-zone gravel packing operation, it is common practice to run a liner string into a cased hole in order to isolate the various zones from one another through use of packers placed between the zones. Such a gravel packing operation and the apparatus therefor is described in U.S. Pat. No. 4,273,190 to E. E. Baker et al, assigned to Halliburton Company and incorporated herein by reference. Inflatable packers, such as are disclosed in the aforesaid patent, are usually employed to isolate the zones from one another and from the remainder of the well bore. However, in certain geological formations, particularly as petroleum wells are drilled to even greater depths, the temperatures exceed those below which an inflatable packer may be employed. This is due to the inability of an inflatable packer employing an elastomeric bladder to withstand high temperatures without leakage past the packer or breakdown of the elastomeric packer components. Similarly, a compression-type elastomeric element packer will not function as these elements will fail under high temperatures. Furthermore, as steam injection becomes more prevalent for enhanced recovery operations in petroleum wells, elastomers will not perform adequately under the temperatures generated by the injection process. The use of non-elastomeric packer elements in known packers presents a problem in initiating the seal of the packer, as non-elastomeric elements generally tend to seal only at higher temperatures, which presents a problem in wells where the initial temperature may be only 150.degree. F. at the packer location, such as in a well in which steam injection may be employed after the liner string is in place.
For example, the packer element disclosed in U.S. Pat. No. 4,281,840 to Harris, assigned to Halliburton Company, comprises packer segments formed of asbestos fibers impregnated with a thermoplastic such as polytetrafluoroethylene (Teflon) and interwoven with Inconel wire. The Inconel wire/asbestos fiber weave provides some resilience to the packer element at high temperatures, while the thermoplastic bridges between the asbestos fiber and Inconel wire, preventing steam or fluid migration through the packer element. However, at low (ambient) temperatures encountered in most non-geothermal wells, this bridging does not take place, and a defective seal results.
Similar problems attend the use of a packer element as disclosed in U.S. Pat. No. 4,258,926 to Upton, which employs particles of asbestos fiber mixed with mica particles, this mixture being confined by a mesh enclosure. This mix is compressed when the packer is set, and is "cured" as the well bore temperature is raised. Again, there is no adequate low temperature seal, as the gaps in the packer element "mix" will not be eliminated until the well bore temperature is substantially raised.
The problems associated with the non-elastomeric packer elements disclosed above were sought to be solved by use of both elastomeric and non-elastomeric packer segments in U.S. Pat. No. 4,296,806 to Taylor et al. A number of different packer elements are disclosed, the general design being a center elastomeric packer element of generally trapezoidal cross-section, with wire mesh-reinforced end elements of various materials, both elastomeric and non-elastomeric. The elastomeric material, and particularly the center ring, provides an initial low temperature seal, and a high temperature seal is sought by the use of a (generally) higher temperature material in the end segments, interwoven with the wire mesh. However, the disclosure indicates that the center segment may liquify at high (unspecified) temperatures, and goes into great detail regarding the role that the wire mesh and backup rings play in preventing extrusion of the elastomers with resultant loss of the seal. Such a design is obviously unreliable for a permanent installation.